I hope everyone realizes that in a super-flat world you only have 4 levels, 2for dirt, 1 for grass, and lastly bedrock. There will be no forests, water, nothing else literally just miles of grass everywhere.
A normal generated world on the other hand, would have all the above and even more...removing and making mountains is just as easy with creative mode enabled.
We are choosing most acceptable option, not favorable, at least i am! Also 2 creative worlds would just complicate things and most players would prefer to use normal generated world, simply to have access to a scenery of forests, snow, mountains, and oceans in their builds.